The present invention is directed to bicycle transmissions and, more particularly, to an internally mounted multi-speed hub transmission for a bicycle.
An internally-mounted multi-speed hub transmission sometimes is mounted to the rear wheel of a bicycle so that the rider can select different gear ratios to vary the pedaling effort. A typical hub transmission includes a hub axle that is mounted to the bicycle frame, a driver rotatably supported to the hub axle for receiving the pedaling force through a sprocket and chain, and a hub shell rotatably supported to the hub axle. A power transmitting mechanism is disposed between the driver and the hub shell for communicating rotational power from the driver to the hub shell through a plurality of power transmission paths, wherein each power transmission path typically produces a unique gear ratio. The power transmitting mechanism ordinarily comprises a planetary gear mechanism including one or more sun gears rotatably supported around the hub axle, a ring gear rotatably supported around the hub axle, a planet gear carrier rotatably supported around the hub axle, and a plurality of planet gears rotatably supported to the planet gear carrier and meshing with the sun gear and the ring gear. The plurality of power transmission paths and the corresponding gear ratios are selected by selectively nonrotatably coupling the various components to each other. For example, one gear ratio may be selected by nonrotatably coupling a sun gear to the hub axle, another gear ratio may be selected by nonrotatably coupling the driver relative to the planet gear carrier, and another gear ratio may be selected by nonrotatably coupling the driver relative to the ring gear. Many such coupling relationships often are possible in a typical hub transmission, thus resulting in a relatively large number of possible gear ratios.
A shift mechanism is usually provided for selecting the plurality of power transmission paths. The shift mechanism may comprise a shift sleeve that surrounds the axle such that rotation of the shift sleeve controls the nonrotatable coupling of the various components. Such a shift sleeve ordinarily is coupled to an actuator member outside the hub, wherein rotation of the actuator member is controlled by a shift control device mounted to the handlebar or by a motor that is electronically controlled by the rider. When the rider exerts a large amount of force on the pedals to accelerate the bicycle quickly, a very large amount of force is applied to the internal components of the hub, thus creating significant resistance to the shifting operation. Such resistance results in excessive manual shifting effort required by the rider or in unacceptable strain on the motor that drives the actuator member.
U.S. patent application Ser. No. 09/522,703 filed Mar. 10, 2000 by the present applicant discloses an apparatus that uses the rotational power of the hub itself to assist the shifting operation when significant drive force is applied to the hub. That apparatus senses when the shift sleeve experiences significant resistance to the shifting operation. Such resistance activates a pawl mechanism coupled to the shift sleeve so that the pawl mechanism engages a ratchet mechanism formed on the inner peripheral surface of the driver. The rotational power of the driver is thus communicated to the shift sleeve, and the shift sleeve is rotated to complete the shifting operation. Because of the firm coupling between the driver and the shift sleeve through the pawl mechanism, there is a possibility that a component in the hub could be damaged if that component is unable to complete the shifting operation. Thus, it is desirable to ensure that such damage does not occur.
The present invention is directed to a bicycle hub transmission having a shift assist function wherein the amount of assisting force is controlled to avoid damage to the hub. In one embodiment of the present invention, a hub transmission for a bicycle comprises a hub axle; a driver rotatably supported to the hub axle; a hub shell rotatably supported to the hub axle; a power transmitting mechanism disposed between the driver and the hub shell for communicating rotational power from the driver to the hub shell through a plurality of power transmission paths; a shift mechanism for selecting the plurality of power transmission paths; a shift assist mechanism for communicating rotational power from the driver to the shift mechanism; and a power control mechanism disposed between the driver and the shift assist mechanism and coupling the driver to the shift assist mechanism for controlling an amount of rotational power communicated from the driver to the shift mechanism.
In a more specific embodiment, an actuator member may be rotatably supported by the hub axle, and a shift control sleeve may be rotatably supported by the hub axle, wherein the shift control sleeve is operatively coupled to the actuator member for rotating in response to the rotation of the actuator member. In this case the shift assist mechanism may communicate rotational power from the driver to the shift sleeve. If desired, the power control mechanism may comprise a first power control member operatively coupled to the driver for rotation in response to rotation of the driver and a second power control member engaging the first power control member. The first power control member may rotate together with the second power control member until the second power control member significantly resists rotation of the first power control member whereupon the first power control member rotates relative to the second power control member. If desired, the first power control member may contact the second power control member, and a power control biasing member may be provided for biasing the first power control member and the second power control member towards each other.